Control units for automotive engines are disposed in a confined space in an environment subject to high ambient temperatures and extreme vibrations. Typically, in order to enhance thermal performance and protection against engine vibrations, a printed circuit board that includes control circuitry is affixed to a rigidizer that may be bent to produce a reduced size module and that also functions as a heat spreader. For example, in an application using a printed circuit board that has a bent flexible circuit, as is used for packaging an electronic control unit, the printed circuit board is mounted to an aluminum rigidizer. The rigidizer provides mechanical support for the printed circuit board and assists in the dissipation of heat generated by components on the printed circuit board by conductively transferring the heat from the components to the underlying rigidizer.
Referring now to FIGS. 1 and 2, an electronic control unit (ECU) 100 of the prior art is illustrated. FIG. 1 is an isometric perspective of an end view of electronic control unit 100. FIG. 2 is a cross-sectional side view of electronic control unit 100. Electronic control unit 100 includes a flexible printed circuit board 104 that is affixed to a rigidizer 106 by use of an adhesive 202. Typically, circuit board 104 has a number of electronic components 102 mounted on the surface of the circuit board. Adhesive 202 is disposed between rigidizer 106 and printed circuit board 104 and functions to secure the printed circuit board to the rigidizer. After being secured to the flat rigidizer, the rigidizer and printed circuit board assembly is folded, resulting in a folded printed circuit board consisting of two main partitions 204, 208 at an angle to each other and connected by a serpentine bent section 206 having multiple tight radius bends 210 with a radius of curvature that is typically less than three millimeters (mm). For example, circuit board 104 may be bent using a Polybent™ process, as described in U.S. Pat. No. 5,998,738, the text of which is hereby incorporated by reference herein.
As the functionality of electronic control units has increased over time, the corresponding circuitry has become increasingly dense and complex. As a result, electronic control units have been migrating from the use of two-layer printed circuit boards, such as printed circuit board 104, to the use of four-layer printed circuit boards. One result of the manufacturing process of four-layer printed circuit boards is a thicker flexible circuit 206, even when the flexible circuit remains a two-layer circuit. Due to the thicker flexible circuit 206, flexible circuit boards are now known to crack and split when bent, resulting a control module that must be discarded. The discarded control modules result in excessive manufacturing costs and waste, especially since the printed circuit boards must be populated with components before being folded.
One solution has been to provide fewer layers in the circuit board at the bend section than in the flat sections. Another solution has been to heat the circuit board during bending. Both of these solutions require additional processing steps in either the manufacturing of the circuit board or the assembly of the electronic control unit.
Therefore, a need exists for a method and apparatus for providing a folding printed circuit board assembly that will not cause splitting and cracking of the flexible circuit.